1. Field of the Invention
The present invention relates to an X-ray analyzing apparatus for radiating a primary X-ray from an X-ray radiating source so as to irradiate a surface of a sample to be analyzed in a direction generally slantwise and for measuring the intensity of a secondary X-ray generated from the sample and, more particularly, to the X-ray analyzing apparatus capable of maximizing the utilization of the primary X-ray from the X-ray radiating source to accomplish the measurement with high precision.
2. Description of the Prior Art
A fluorescent X-ray analyzing apparatus has long been well known in the art, an example of which is shown in FIG. 10. The prior art fluorescent X-ray analyzing apparatus shown in FIG. 10 is so designed that a primary X-ray 3 generated from an X-ray radiating source 4 comprised of, for example, an X-ray tube is irradiated upon a sample 1 fixed on a sample support 2 and the intensity of a fluorescent X-ray 5 generated from the sample 1 can be measured by a detecting means 6. In this structure, the primary X-ray 3 is deemed to emit in a conical shape with respect to a center line (a principal radiating direction) represented by an axis T of the X-ray radiating source 4. In order to position the X-radiating source 4 so close to the sample that the sample 1 can be irradiated with an increased intensity of the primary X-ray 3 while assuming the fluorescent X-ray 5 generated from the sample 1 to be detected by the detecting means 6 without any interference with the X-ray radiating source 4, the principal radiating direction of the X-ray radiating source 4 is inclined at a predetermined angle, for example, 65.degree., relative to a surface 1a of the sample 1 confronting the X-ray radiating source 4 so that the sample 1 can be irradiated slantwise by the primary X-ray 3.
In this known fluorescent X-ray analyzing apparatus, a sample support 2 having the sample 1 fixed thereon is so positioned that the axis T of the X-ray radiating source 4 can reach and align with the center of a measuring area of the sample 1 (a measuring area N of the surface 1a of the sample 1 in the example shown in FIG. 10). In the illustrated example, the detecting means 6 includes a spectrometer or a monochromator 8, an X-ray detector 9, and a goniometer 18 for rotating the monochromator 8 and the detector 9 while the monochromator 8 and the detector 9 are kept in a predetermined relationship. The monochromator 8 and the detector 9 are drivingly associated with each other by means of the goniometer 18 so that respective angles of rotation of the monochromator 8 and the detector 9 with respect to an imaginary axis extending through the center of a surface 8a of the monochromator 8 at right angles to the plane of the drawing depicting FIG. 10 can assume a relationship of 1:2. With this construction, the intensity of the fluorescent X-ray 5 generated from the measuring area N of the sample 1 can be monochromated and measured for each wavelength. In other words, the prior art fluorescent X-ray analyzing apparatus employs an optical system of a so-called parallel beam method and is, in most cases, provided with a paralleling slit such as, for example, a solar slit, positioned between a field-limiting slit 19, as will be described later, and the monochromator 8 and between the monochromator 8 and the detector 9.
A slitted plate 20 is disposed between the sample 1 and the detecting means 6 and is formed with the field-limiting slit 19 having a slit size corresponding to the size of the measuring area N of the sample 1 and operable to pass therethrough only a fluorescent X-ray 5 generated from the measuring area N of the sample 1 while cutting off the fluorescent X-ray emanating from surroundings including the sample support 2. Although the fluorescent X-ray component 5 emits in all directions from the measuring area N of the sample 1, the slitted plate 20 and the detecting means 6 are so positioned that the amount of the fluorescent X-ray component 5 passing through the field-limiting slit 19 and subsequently incident upon the detecting means 6 can be maximized.
As discussed above, the sample 1 is so positioned that the axis T of the X-ray radiating source 4 can reach and align with the center of the measuring are an N of the sample 1. There is no specific reason for positioning the sample 1 in this way, but it is based on the fact that where the primary X-ray 3 from the X-ray radiating source 4 is to be irradiated towards the surface 1a of the sample 1 in a direction perpendicular thereto, the radiation can be maximized along the axis T of the X-ray radiating source 4.
When a simulation test was conducted to determine the pattern of distribution of radiation intensities of the primary X-ray 3 on an imaginary plane A to be radiated which includes the surface 1a of the sample 1 and a surface extension 1aa from the sample surface 1a, the inventor has found that where the primary X-ray 3 from the X-ray radiating source 4 was irradiated towards the sample surface 1a in a slantwise direction, as shown a curve B in FIG. 11, the radiation intensity distribution B did not attain a maximum value at a position C where the axis T of the X-ray radiating source 4 reaches the imaginary radiation plane A, but attained a maximum value at a peak position M displaced from the position C in a direction conforming to the direction of inclination of the X-ray radiating source 4, with the curve B consequently exhibiting an asymmetric distribution with respect to left and right sides. It has also been found that the reason therefor is because a left side of the position C on the imaginary radiation plane A is closer to the X-ray radiating source 4 than the position C on the same imaginary radiation plane A This result of the simulation test has been confirmed by a series of experiment conducted wherein an X-ray sensitive film was placed on the imaginary radiation plane A and exposed to the primary X-ray 3.
Accordingly, where, for example, a measuring area of the sample 1 is relatively small, say, having a diameter D5, that is, where the entirety of the surface 1a of the sample 1 which is small as shown in FIG. 10 is chosen to be the measuring area, or where although the sample 1 is not so small, only a small portion of the surface 1a of the sample 1 is chosen to be a measuring area, positioning of the sample 1 so that the axis T of the X-ray radiating source 4 can reach and align with the center of the measuring area such as hitherto practiced will result in utilization of an hatched area in FIG. 11 where the intensity of radiation of the primary X-ray 3 from the X-ray radiating source 4 is not maximum and, consequently, measurement with a sufficiently high accuracy cannot be accomplished. It is to be noted that numerical values depicted along the axis of abscissa in FIG. 11 are arbitrily chosen values to show relative relations and are not actual dimensions and, therefore, they have no unit.